If your question is current. You can calculate the minimal volume of RE reaction as follows: Theoretically one unit of DdeI is defined (according to New England Biolabs) as the amount of enzyme required to digest 1 µg of λ DNA in 1 hour at 37°C in a total reaction volume of 50 µl. Practically you sould use much more enzyme, I would first try to use 50 U for overnight for 50 µg PhiX174 DNA. 50 U DdeI is in 5 µl storage volume (in DdeI produced by New England Biolabs). The volume of enzyme must be maximally 10% of the total volume because of star activity, therefore, the minimal volume of the reaction should be 50 µl in this case. Otherwise, you won't make a mistake, if you use a larger total volume. If you want to use more concentrated DNA for the labelling step, you can extract the DNA with a kit, and then, DNA can be solved again in a smaller volume.